Aircraft main engines not only provide propulsion for the aircraft, but in many instances may also be used to drive various other rotating components such as, for example, generators, compressors, and pumps, to thereby supply electrical, pneumatic, and/or hydraulic power. However, when an aircraft is on the ground, its main engines may not be operating. Moreover, in some instances the main engines may not be capable of supplying power. Thus, many aircraft include one or more auxiliary power units (APUs) to supplement the main propulsion engines in providing electrical and/or pneumatic power. An APU may additionally be used to start the main propulsion engines.
An APU is, in most instances, a gas turbine engine that includes a combustor, a power turbine, and a compressor. During operation of the APU, compressor draws in ambient air, compresses it, and supplies compressed air to the combustor. The combustor receives fuel from a fuel source and the compressed air from the compressor, and supplies high energy compressed air to the power turbine, causing it to rotate. The power turbine includes a shaft that may be used to drive the compressor. In some instances, an APU may additionally include a starter-generator, which may either drive the turbine or be driven by the turbine, via the turbine output shaft. Some APUs additionally include a bleed air port between the compressor section and the turbine section. The bleed air port allows some of the compressed air from the compressor section to be diverted away from the turbine section, and used for other functions such as, for example, main engine starting air, environmental control, and/or cabin pressure control.
Although most APUs, such as the one generally described above, are robust, safe, and generally reliable, some APUs do suffer certain drawbacks. For example, when some APUs are operated at part power, the surge margin of the APU compressor, or at least one or more stages of the compressor, can be reduced. Thus, in many instances APU compressors include a diffuser that is not optimally designed for operations over the entire operational envelope of the APU, which can result in reduced operational efficiency (e.g., increased specific fuel consumption (SFC)) of the APU. Alternatively, some APUs include a surge valve, or mechanically variable diffuser vanes, which allow the geometry of the vanes within the diffuser to be varied. However, these systems and methods can be inefficient, complex, and costly.
Hence, there is a need for a system and method of controlling the surge margin, and thus improving the overall operational efficiency, of a compressor, that is more efficient, less complex, and less costly than existing systems and methods. The present invention addresses one or more of these needs.